Refers to the silver foil bag accociated with cask wine.
The origin of the term "Silver Lady" is not well documented, however, has most likely been borrowed from the stories of sailer's encounters with mermaids/sirens, where the beautiful songs of these seductresses would lure the sailers to the rocky shallows of an island cove and ultimately their demise.
The metaphor relates back to the Silver Lady in that, like the aformentioned seductresses, she draws her victims in with her affordable price and shiny appearance. Seemingly all is well and good, desired intoxication is achieved and everyone is happy. That is until consumption thresholds are exceeded resulting in the obvious, or in the unlikely case that you survive till morning (assuming this is a night scene), the hangover to end all hangovers must be endured.
Drunk & Disorderly #1: "What is that alluring melody I hear on the wind?"
Drunk & Disorderly #2: "Why, it's the Silver Lady calling!"